Generic drugs in the US:
-have to be absolutely identical in the active ingredient
-be identical in strength, dosage form, and route of administration
-have the same use indicators
-meet the same batch requirements for identity, strength, purity, and quality
The difference of the 20% either way ONLY pertains to the inactive ingredients. Every generic medication must have the exact same ACTIVE ingredient.
This is set by laws and the FDA to ensure safety. Just like a generic heart medication or cancer medication could never say they have 20mg of the active ingredient and have that not be true.
It's the "fillers" that can be different in the meds as they do not effect the integrity of the medication.
Many people have reactions to the inactive ingredients and seem to find one generic over another better. Or brand over generic.
For myself, being in PM for 11 years and having taken every single opiate from Vicodin to Fentanyl patches and all in between...I have never found a difference in my generic meds or felt they work any less.
So, you may just be having some sort of reaction to the fillers in the Watson brand. You can certainly speak with your Dr. or Pharmacist to see if you can find the ones that seem to suit you better.